Recent studies with Aspergillus have shown that marked differences in induced mutation frequencies after apparently replicate mutagenic exposure are mainly due to the presence of varied amounts of self-inhibitors of germination in the conidia at the time of mutagenic treatment. With some mutagens, removal of these compounds has also affected the qualitative mutational response of thirteen genes which may be simultaneously studied in this organism, Aspergillus nidulans. It would be very interesting to determine whether such substances are also present in the conidia of Neurospora crassa and to investigate the effect these substances might have on the processes of mutation induction. Such a program, it is hoped, will increase the confidence with which the mutagenic data obtained with fungal test systems can be meaningfully extrapolated to man. Isolation of inhibitory substances might also be very useful in extending the presumptive mutagenic treatment times which may be given to fungi in a host mediated assay or microsomal activation situation, hence increasing the detection capability of this organism.